Direct current converter



Oct. 8, 1935. w DA LENBA H 2,016,628

I DIRECT CURRENT CONVERTE Filed Feb. 10, 1935 2 Sheets-Sheet 2 F/HSESH/FIER /n ventor:

electrically by a Figure 1 of the accompanying the back E. M. F.

Patented Oct. 8 1935 I PATENT orrice 2,016,628 nmno'r CURRENT CONVERTER Walter Dallenbaci, Barlin-Charlottenburg,

ermany Application February 10,

1933, Serial No. 656,180

a In Germany February 10, 1932 1 Claims. (01. 171-91) My invention relates to an arrangement for converting direct current into alternating current by means or grid-controlled discharge vessels functioning as inverters. The said vessels may be filled with gas or vapour, may have a mercury or hot cathode and may be of any desired phase number and type. v

For a better understanding of' the essential features'oi my invention, the theoretical principles which here come into consideration will first be discussed.

The direct voltage provided by a mercury vapour rectifier diminishes with increasing load until it reaches about zero,chiefly because of the stray reactances in the alternating current circuit. If the ignition point of the anode currents is retarded by control grids, and such retardation is kept constant with increasing load, the external characteristic of. the rectifier is shifted substantially parallel to itself towards lower values of the voltage.

Fig. 1 is a graphic representation of the characteristics of an inverter. I

Fig. la is a graphic representation of the instantaneous values of the alternating current voltages applied to three successive inverter anodes.

Fig. 2 is a graphic showing of the characteristics of an inverter but with a change in sign f the voltage.

Fig. 3 is a circuit diagram illustrating one form of my invention.

Fig. 4 is a circuit diagram illustrating a modification. I i

f Fig. 5 is a circuit diagram illustrating another orm.

The conditions mentioned above are shown in drawings where G is the direct current voltage on the D. C. side of the rectifier and J is the direct current. The normal external characteristic of the rectifier installation is shown at I; while at 2 is shown the characteristic, which is shifted substantially parallel to itself towards lower values of the potential and which is obtained from I when the ignition point of the anode currents is retarded certain phase angle. If the retardation of ignition is made greater and greatv er, forinstance when the rectifier. is under noload operation, a point both the direct current and the voltage of-the rectifier will be zero. If a reversed or counter E. M. F. be provided in the direct current circuit, of reversed polarity, on further increasing the retardation of ignition, may be will be reached when this limit line increased to a value which is equal and opposite to the no-load voltage of the device when acting as a rectifier. From the moment of reversal'oi the polarity, the rectifier functions as an inverter and now supplies energy to the alternating current circuit. The back E. M. F. can at the most be raised to a value -60, where G is the no-load voltage during normal rectifier operation without control grids. The question now arises, up to what direct current valuesis it posl0 sible to load the rectifier in the. region in which it functions as an inverter? 1 Mathematical analysis shows thatthe characteristic which is obtained under the influence of a predetermined constantretardation of ignition holds good as far as the limit 3 shown in dot-and-dash line in Figure 1. This limit has been found with a good degree of accuracy to be the mirror image of the characteristic I relatively to the current axis :J as mirror axis. It 20. if an attempt is made to load apparatus is over-controlled and falls out of step like a synchronous machine owing to the formation of internal short circuits.

These statements cover the entire range of possible cases of loading which is attainable with a controlled rectifier. 2. 4,5 and 6 are characteristics which are all produced from the characteristic I by substantially parallel shifting and by increasing the retardation of ignition in each instance. Each of these characteristics corresponds to possible loading conditions only before attaining its point of intersection with the limit 3. If an attempt is made to carry the loading beyond 3, the device will fall out of step. This phenomenon may be explained as follows:

In Figure 1 let E be the back E. M. F. impressed in the direct current circuit during inverter operation; this voltage E does not .by any means fix the condition of the inverter; but according to the choice of'the' grid' control, it provides an entire series of possible loading conditions which are the voltage E.

to the minimum retardationof ignition, and the characteristic 5 to the maximum retardation of ignition. If, now,.the retardation of ignition is so regulated that the condition of the inverter moves from the characteristic 2 through the characteristic 4 to the characteristic 5, the intensity of the current flowing through it diminishes from the value J 2, which is the maximum possible with the voltage E, to J4 and finally to J 5:0, that is to istic 5 corresponds to the case in which the voltage E appears as the no-load voltage of the inverter. 7 It, now, this voltage E is increased, the case arises that E assumes Just the value Go, Go being the no-load voltage of the rectifier. voltage E=-Go is only possible for quite a defigrids in which the load for attaining the step limit coincides with no-ioad, that is to say, a condition of loading which cannot be realized in practice.

Let the control grids of the anode I be released at the point oi t1.

lags by 2/61r=60" behind t1, the potential 01' the grid coresponding to the anode II is changed to a positive value and the anode II ignites. At this moment the grid 01' anode I is changed to a negasay, with increasingly delayed moment eat-- tinction t: or the current ii, the said moment of extinctiont: oi the current in anode I finally occurred at the time t4. currentless interval t:=t4, corresponds in Figure 1 to the direct hich the working point on the horizontal line E just attains the step limit.

In actual practice, it is impossible to approach conditions between the curves I and 1 and above apparatus as a rectifier, while the region between the axis J and the curve 1 below the axis corresponds to the conditions as an inverter. curve 1 may be termed the "true step limit.

Figure 2 again shows the characteristics for limit.

It, now, an inverter 0! the present type is fed from a direct current network, there is a simple and reliable means of preventing the apparatus from coming out of step, even when there is overcontrol of the grids. It is merely necessary to impart to the device feeding the direct current circuit an external characteristic, for example curve 8, which extends below the step limit 1 of the inverter. This external characteristic then renders it impossible, in the event oi over-control or" the inverter, for conditions in the immediate vicinity of the step limit to be attained.

possible range of loads between no-load and short-circuit.

* when the external characteristic 8 of the rectifier circuit.

The :external characteristic may be influenced in anydesired manner. The external charactersource of direct current, the adaptation may be attained by means ofthe control grids.

Figures 3 to 5 of the accompanying drawi gs which, like the claims, term a part 01' my appli ation, show various constructional examples of my invention. the axis J corresponds to all the conditions 01 the Figure 3 showsa circuit arrangement in which connected to the the external characteristic is influenced by means of conductor elements.

Figure 4 shows a circuit arrangement which Q only differs from that according to Figure 3 by the 1 fact that the conductor elements provided are made adjustable;

Figure 5 shows a circuit arrangement in which the external characteristic is influenced by means of control grids.

The rectifier I3 supplying the direct current is alternating current supply network l2 through the transformer H.

The device 13, in this example is shown as a mercury vapour rectifierwith a cathode 20. It is possible, however, to employ any other suitable device for supplying direct curren fier- I3 operates on the direct current circuit l1 to which an inverter I4 is connected in known manner. The said inverter is likewise shown in the form of a mercury vapour discharge vessel with a cathode 22, but it is also possible to employany other type of rectifier. The inverter is provided with control grids IS, the grid potential of which may be varied by operating the adjusting lmob 2! of the control device Is. As is known, the output given by the inverter depends upon the adjustment of the grid potential. The inverter feeds-the alternating current network l9 through the transformer l8.

' The circuits and the control devices thus described agree with the known constructions, so

that persons versed in the art are acquainted with them, and hence a detailed description of the circuit will not be given.

In order now to be able to influence according to the invention the external characteristic of the rectifier i3 feeding the direct current circuit it, reactances iii, in which during load there occurs a drop in voltage reducing the direct current voltage, are inserted in the alternating cur rent side of the rectifier. The said reactances are so dimensioned and adjusted as to give the desired external characteristic of the rectifier it.

The operation of the system shown in Figure 3 is as follows: The polyphase transformer ii, having the primary and secondary winding thereof connected to the power supply line 92 and the reactarice units iii, respectively energizes the rectifier 13. The output characteristic of this rectifier is adjusted by proportioning the impedance characteristic of the anodeto cathode paths, the reactances it and the transformer i i in such a way that the output current and voltage characteristic of the rectifier is defined by the curve 8 of Figure 2. The output current and voltage characteristic of the rectifier is thus adjusted to permit the operation of the inverter, fed by the rectifier, over a wide range of current and voltage values. This is accomplished by adjusting the rectifier output characteristic so that it varies in a predetermined manner as the inverter load characteristic varies. v v

The output direct current from the rectifier is impressedupon the cathode and anode circuit of direct current flows from.

the inverter M. The the cathode 22 to the anodes Ma, through the primary windings @812, Nb and lac of the transformer l8. The direct current from the rectifier i3 is caused to fiow through the dif- MD and c ferent windings Ilia, I81) and I80 successively, and

through the corresponding anode cathode circuits of the inverter and thus set up alternating potential in the corresponding secondary windings of the transformer. Inasmuch as the alternating current lines l9 operate at a definite frequency The rect1- interrupted to obtain proper 'At the interval 155 the potential anode Ma would the inverter M must be adjusted to maintain the alternating current derived therefrom in proper phase and voltage relation with the alternating current transmitted over. said lines. This is accomplished by proper commutating action in the inverter by the use of the grid electrodes l5a, I51) and I50 which function to initiate the electric discharge between the cathode and the anodes Ha, Mb and I40, respectively. When the current flow between anode l5a and the cathode 22 is initiated by impressing the the grid I501, the primary winding l8a is-energized and an electromotive force is induced into the secondary winding corresponding thereto. secondary induced voltage must be sufiicient to overcome the line voltage existing in the secondary winding in order that power may be fed to the line I!) from the inverter. The potential of the grid electrode i511. is adjusted to prevent the circuit of the anode 14a from being energized until the winding lBa of the transformer is energized sufficiently to assure the feeding of power from the inverter into the line l9.

After the current flow through the anode Ma and primary tained for a predetermined fraction ofthe alternating current cycle of the line current of the line IS, the output circuit of the anode Mb is energized. Simultaneously the current flow through the output circuit of the anode Ma is commutating action. The current through the circuit of the anode MI) is started by decreasing the negative potential on the; corresponding grid electrode l5b. The potential of the grid electrode l5a is at the same time increased negatively. As a result the anode Nb assumes the load and the anode Ma. becomes inactive; The interval during which the anode Ma carries the major part of the load is defined by the interval between ti and t2 (Figure 1a). At t; the potential of the grid electrode l5b is changed to a positive value and the potential of grid electrode ifm is changed to a negative value. The shift in load from anode Ma to anode Mb is therefore initiated at is. The interval between ta and t3 defines the angle of overlap during which both anodes Ma and Mb carry some of the load current, however, during this interval the anode Mb is assuming the load from the anode Ma and consequently the current through the anode Mb is increasing and that through the anode Ma is decreasing.

At the interval 134 the potential of the grid electrode i5?) is changed to a negative value and that of the grid electrode P50 is changed to a positive value while the grid electrode l5a is maintained negative. The anode He, therefore, begins to assume the load from the anode Mb at this point.

of the grid electrode I50 isv changed to a negative value and the load'is shifted from the anode Me to another anode. In the case of /a three anode tank the start to assume the load at this point, however, in a six anode tank the anode connected to the next phase would assume the load. The operation of the systems shown in Figures 4 and 5 is similar to that of the system illustrated in Figure 3.

In the circuit arrangement according to Figure 4, variable reactances 23 adjustable, by means of sliding contacts, according to the existing working conditions are provided in place of the fixed reactances iii. In other respects, Figure 4 corresponds exactly to Figure 3.

Figure 5 shows a circuit arrangement in which proper potential on trol grids in 4 the external characteristic oi the rectifier I3 is influenced by means of control grids 25. The

grid potential is .controlled by means of a control device 26, which is connected to the alternating current network l2 and also, through a shunt 21, to the direct currentcircuit l1, The construction of these grid control devices persons versed in the art,-so that they will not be described indetail here.- i

It is furthermore possible to influence the external proportioning the said device. When a rectifier is employed, voltage must be suitably selected.

The essential feature of my invention is solely the fact that the external characteristic of the device feeding the direct current circuit is influenced according to the invention, but not the method and means employed for effecting such influencing. My invention is thereforenot to be limited to the constructional examples given in the foregoing.

I claim:

1. In an arrangement for converting direct current into alternating current comprising, a direct current circuit, a discharge device for feed ing'said direct current circuit, a second discharge device ted by said directcurrent circuit for converting the .direct current supplied into alternatng current, an alternating current network on hich said second discharge device operates, consaid second discharge device, means r varying grids, and

between no-load and short circuit.

2. In an arrangement for converting direct current into alternating current comprising, a direct current circuit, an electrical discharge device for feeding the said direct current circuit, a second electrical discharge device fed by the said direct current circuit and adapted to convert the direct current supplied thereto into alternating current, an alternating current network on which therefore, its alternating I reactances connected in the alternating current side 01' 5. A method of preventing the falling out of step of a grid control inverter filled with a gaseous medium, connected in circuit for converting WALTER DKLLENBACH.

preventing the falling out of 

